This invention relates to automotive brake master cylinder vacuum booster devices and, more particularly, to improvements in those of the type comprising a booster shell, an axially slidable booster piston accommodated in the booster shell, a piston diaphragm the inner and outer peripheral edges of which are respectively fixed to the rear surface of the booster piston and the inner peripheral surface of the booster shell, a front side first working chamber and a rear side second working chamber defined in the booster shell by the booster piston and piston diaphragm, said first working chamber commumicating with a vacuum source, said second working chamber selectively communicating with the first working chamber or the external atmosphere through a control valve, an input rod arranged opposite to the booster piston for movement toward and away from the latter and connected to the control valve so as to produce a pressure difference between both the working chambers effective to cause the booster piston to follow forward movement of the input rod, tie rods extending through the booster piston for connection of the front and rear walls of the booster shell, and a flexible bellows arranged between the tie rod and booster piston for enabling the booster piston to be operable.
When the above-mentioned booster device is operated, the booster shell can be protected from effect of the forward thrust loaded to the master cylinder by transmitting the load to the automobile body through the tie rods, so that such rigidity high enough to bear the thrust loading is not required to be given and, what is advantageous, the booster shell can be made less in weight by being formed of thin steel sheets, synthetic resins or the like.
As mentioned above, in case of said booster device, the bellows is arranged between the booster piston and therethrough passing tie rod so that, unless the booster piston is correctly positioned in the circumferential direction with respect to the booster shell, the operation of the booster piston is likely to be hindered by the interference of the booster piston to the tie rod and also the bellows is subjected to the twist which may cause the decrease of the durability thereof.
Portion of the piston diaphragm located between inner peripheral surface of the booster shell and outer peripheral surface of the booster piston is formed turned so as to make the diaphragm section of U-shape projecting into the first working chamber, and the booster piston can be moved forward and backward with the movement of the U-shaped turning part. However, if a brake pedal is depressed and the booster piston is advanced while the vacuum pressure is not yet accumulated in the first working chamber, that is, when the vacuum pressure is not produced in the vacuum source, a pipe line communicating the first working chamber with the vacuum source goes wrong or else, the air inside the first working chamber is not sufficiently discharged into the vacuum supply source due to the resistance inside the pipe line or the like. As a result, the remaining air inside the first working chamber is compressed and the air pressure of the second working chamber is exceeded by that of the first working chamber so that the piston diaphragm is swelled out to the second working chamber side by the aforesaid pressure difference and the said U-shaped turning part is likely to be turned over. Application of such an unnecessary pressure to the piston diaphragm is not desirable since the durability of the piston diaphragm is thereby decreased.